1. Field of the Invention
The present invention relates to a zoom lens and an image pickup device, and particularly to a zoom lens and an image pickup device that achieve both a wider angle of view and a smaller size while having a high variable power ratio.
2. Description of the Related Art
Recently, image pickup devices such as digital still cameras, digital video cameras and the like have been miniaturized and spread widely for home use. In these image pickup devices, as image pickup elements are miniaturized, miniaturization of an entire lens system and a wide-angle zoom lens having a high variable power ratio and high performance are desired.
It is generally known that a so-called inner focus type zoom lens used in video cameras and achieving focus by moving a lens group other than a first lens group disposed nearest to an object side facilitates miniaturization of an entire lens system and offers image forming performance suitable for an image pickup element having a large number of pixels.
A mainstream system of such an inner focus type zoom lens is such that a first lens group and a third lens group are fixed groups, a second lens group is moved in a direction of an optical axis mainly to vary power, and a fourth lens group is moved in the direction of the optical axis to correct a focal position resulting from power variation and perform focusing (see Japanese Patent Laid-Open No. 2009-175648 referred to as Patent Document 1 hereinafter, for example).
However, in the zoom lens described in Patent Document 1, the first lens group is formed by arranging a cemented lens of a negative lens and a positive lens and a positive lens in order from an object side to an image side, and a half angle of view at a wide-angle end is only about 30°. Thus, when a wider angle of view is to be achieved, the height of off-axis light rays incident on the first lens group is raised, and the effective diameter of the first lens group is increased.
Accordingly, a first lens group in a zoom lens of a four-group inner focus type of a positive group, a negative group, a positive group, and a positive group is composed of five lenses to achieve both a wider angle of view and a smaller size while having a high variable power ratio for diverse variations (see Japanese Patent No. 4007258 referred to as Patent Document 2 hereinafter, for example).
In the zoom lens described in Patent Document 2, a wider angle of view and a smaller size are achieved with a four-group configuration by making the first lens group have a five-lens configuration.